The present invention relates to a method and apparatus for pre-concentrating analytes and removing moisture in a sample. More specifically, the invention relates to an adsorbent trap for use with chromatographic columns, chromatographic injectors, and headspace samplers.
Chromatography is essentially a physical method of separation in which constituents of a test sample in a carrier gas or liquid are adsorbed or absorbed and then desorbed by a stationary phase material in a column. A pulse of the sample is introduced into a steady flow of carrier gas. At the end of the column, the individual components are more or less separated in time. Detection of the gas provides a time-scaled pattern that, by calibration or comparison with known samples, indicates the constituents of the test sample. An example of the process by which this occurs is described in U.S. Pat. No. 5,545,252 to Hinshaw.
The value of using a separate, heated device for receiving the sample and subsequently introducing it into the column has long been recognized. One such device is disclosed in U.S. Pat. No. 4,038,053 to Golay, which describes using a chromatographic injector for receiving the sample, heating it, and subsequently injecting it into a chromatographic column. Such a device is desired because higher sample equilibrium temperatures can result in much larger chromatographic peaks. A disadvantage of such devices, however, is that such temperature increases may also increase the concentration of other material that detrimentally affects the sensitivity of the system, such as water.
To remedy this problem, numerous assemblies have been suggested to pre-concentrate analytes in a sample and remove moisture therefrom prior to introducing the sample into a chromatographic column. For example, U.S. Pat. No. 5,612,225 to Beccanti, U.S. Pat. No. 4,245,494 to Legendre, and U.S. Pat. No. 2,813,010 to Hutchins disclose a means for removing water from a sample prior to introducing the sample into a chromatographic column by first passing the sample through an anhydrous substance, which absorbs the water. However, because the anhydrous substance absorbs the water, rather than adsorbing the analyte and allowing the water to be purged from the system, repeated use of the anhydrous substance is likely to be limited and require frequent replacement.
Several assemblies have been suggested which utilize an adsorbent trap, which adsorbs the analytes while allowing water to pass through. For example, U.S. Pat. No. 6,223,584 to Mustacich discloses the use of an adsorbent trap in a pre-concentrator assembly for pre-concentrating analytes in a sample prior to introducing the sample into a chromatographic column, which device comprises a tube containing an adsorbent material. However, a disadvantage of this arrangement is the dead volume that exists between the adsorbent bed and the chromatographic column, which is undesirable because, at typical column flow rates, excessive peak broadening results.
U.S. Pat. No. 5,814,128 to Jiang discloses the use of an adsorbent trap in conjunction with a separate water management device, which device removes water from a sample prior to entry into a chromatographic column via the swirling motion caused by a threaded (or other non-smooth geometrically shaped) bore in the device. Similarly, U.S. Pat. No. 4,293,316 to Block discloses the use of an adsorbent trap in conjunction with a membrane separator device, which device removes water from a sample prior to entry into a gas analyzer. However, rather than optimizing the utility of the adsorbent trap itself as a means for analyte pre-concentration and moisture elimination, these assemblies each require a separate device in addition to the trap, which not only creates additional manufacture and maintenance costs, but also does not solve the aforementioned problem of excessive volume between the adsorbent bed and the chromatographic column.
One means of introducing a sample containing an analyte into a chromatographic column is known as xe2x80x9cheadspace sampling.xe2x80x9d In conventional headspace sampling, sample material is sealed in a vial and subjected to constant temperature conditions for a specified time. Analyte concentrations in the vial gas phase should reach equilibrium with the liquid and/or solid phases during this thermostatting time. The vial is subsequently pressurized with carrier gas to a level greater than the xe2x80x9cnaturalxe2x80x9d internal pressure resulting from thermostatting and equilibration. Then the pressurized vial is connected to the chromatographic column in such a way as to allow for the transfer of a portion of the vial gas phase into the column for a short period of time. Such a system is disclosed in U.S. Pat. No. 5,711,786 to Hinshaw, which describes using a chromatographic injector between the vial and the chromatographic column. However, the use of such devices presently known in the art, including chromatographic injectors, for headspace applications results in the same disadvantages previously mentioned for introducing samples into chromatographic columns generally.
What is desired, therefore, is a method and apparatus for pre-concentrating analytes and eliminating moisture in a sample prior to introducing the sample into a chromatographic column. What is further desired is a method and apparatus for pre-concentrating analytes and eliminating moisture in a sample in a chromatographic injector. What is also desired is a method and apparatus for pre-concentrating analytes and eliminating moisture in a sample in connection with a headspace sampler.
Accordingly, an object of the present invention is to provide an analyte pre-concentrator for gas chromatography that permits temperature increases without detrimentally affecting the sensitivity of the chromatographic system.
It is a further object of the present invention to provide an analyte pre-concentrator for gas chromatography that provides a moisture trap permitting moisture to be purged from the chromatographic system so that the trap may be used for multiple injections.
It is yet another object of the present invention to provide an analyte pre-concentrator for gas chromatography that provides a moisture trap resulting in little or no dead volume between the trap and a chromatographic column, thereby decreasing excessive peak broadening.
It is still another object of the present invention to provide an analyte pre-concentrator for gas chromatography that provides a moisture trap without the addition of a separate device, thereby decreasing manufacturing and maintenance costs.
It is still another object of the present invention to provide a method for pre-concentrating analytes in a gas chromatographic system that achieves the objects listed above.
To overcome the deficiencies of the prior art and to achieve at least some of the objects and advantages listed, the invention comprises a chromatographic injector, a liner located inside the injector, the liner having an inlet and an outlet, wherein the inner diameter of the outlet is smaller than the inner diameter of the inlet, and an adsorbent located inside the liner. Preferably, the injector is temperature controllable.
In another embodiment, the invention comprises a tube having an inlet and an outlet, wherein the inner diameter of the outlet is smaller than the inner diameter of the inlet, an adsorbent located inside the tube, and a gas chromatographic column coupled to the tube. Preferably, the invention further comprises a heating device for heating the tube.
In another embodiment, the invention comprises a headspace sampler, a tube coupled to the headspace sampler, the tube having an inlet and an outlet, wherein the inner diameter of the outlet is smaller than the inner diameter of the inlet, and an adsorbent located inside the tube. Preferably, the invention further comprises a heating device for heating the tube.
In a preferred embodiment, the invention further comprises a column isolating accessory, with which a chromatographic column can be temporarily isolated from the tube or liner.
The invention also relates to a method comprising the steps of setting the pressures of a main carrier gas inlet and an auxiliary carrier gas inlet such that a column is isolated from substances flowing through a tube, introducing a sample mixture into the tube, such that the mixture passes through an adsorbent, which adsorbs the analytes, and is vented from the chromatographic system, introducing additional carrier gas into the tube, such that moisture is dry purged from the adsorbent, setting the pressures of the main carrier gas inlet and the auxiliary carrier gas inlet such that the column is no longer isolated from substances flowing through the tube, and heating the tube, such that the analytes adsorbed by the adsorbent are desorbed into the column.